1. Field of the Invention
The present invention relates to an electrically heated catalytic converter having a substrate of a catalyst formed as a scroll-like cylindrical laminated assembly of thin metal sheets and, more specifically, relates to a support construction for such a substrate.
2. Description of the Related Art
An exhaust gas purification device that utilizes a three-way reducing and oxidizing catalytic converter disposed in the exhaust passage of an internal combustion engine is commonly used. Generally, the catalyst used in such converters is able to purify pollutants such as HC, CO and NO.sub.x in the exhaust gas only when the temperature of the catalyst becomes higher than a certain temperature, i.e., the catalysts in the converter do not work when the temperature of the catalyst is lower than an activating temperature.
To shorten the time required for the catalyst to reach the activating temperature after the engine starts, electrically heated catalytic converters are used. Usually, electrically heated catalytic converters have metal substrates, and the catalyst is heated during engine start by feeding an electric current through the metal substrates, i.e., by using the metal substrates as electric heaters.
An electrically heated catalytic converter of this type is disclosed, for example, in Japanese Unexamined Patent Publication (Kokai) No. 4-203416. The electrically heated catalytic converter disclosed in JPP '416 has a substrate formed as a scroll-like cylindrical laminated assembly of thin metal sheets.
More specifically, the substrate in JPP '416 comprises a thin plain metal sheet and a thin corrugated metal sheet both provided with insulating coatings on the surfaces. The plain metal sheet and the corrugated metal sheet are placed one upon another and wound around a common center electrode so that the plain metal sheet and the corrugated metal sheet form a scroll-like cylinder of laminated metal sheets. The cylindrical laminated assembly is accommodated in a cylindrical metal casing. The center electrode extends from the laminated assembly in the direction of the central axis of the laminated assembly and is bent toward a radial direction so that the electrode penetrates the wall of the casing. The center electrode is fixed to the casing at the portion where it penetrates the wall of the casing, in such manner that the electrode and the casing are electrically isolated. The cylindrical outer surface of the laminated assembly is connected to the casing in such manner that the laminated assembly and the casing are electrically connected.
When electric voltage is applied across the center electrode and the metal casing, electric current flows through the metal sheets in the laminated assembly. The metal sheets are heated by this electric current and reach the activating temperature of the catalyst a short time after the engine starts.
In a catalytic converter equipped with the substrate disposed in the casing, as disclosed in JPP '416, the substrate must be fixed firmly to the casing to protect the substrate from damage caused by vibration during the operation of the engine. However, since the substrate and the casing are exposed to high temperature exhaust gas, if the substrate is fixed to the casing, an excessive stress may be exerted on the substrate due to the difference in the thermal expansion of the substrate and the casing during engine operation.
To avoid this problem, Japanese Unexamined Patent Publication (Kokai) No. 62-30533 discloses a support construction for the substrate which is capable of absorbing the difference in the thermal expansion of the substrate and the casing. The support construction disclosed in the JPP '533 is applied to a substrate which has no center electrode. In this support construction, a plurality of collar like separating members are disposed between the substrate and the casing. The outer circumferences of the separating members are fixed firmly to the inner surface of the cylindrical casing and the inner circumferences of the separating members contact the outer surface of the substrates while allowing an axial sliding movement of the substrate.
The annular space defined by the separation members is filled with an insulating material (such as quartz sand) which acts as a buffer between the substrate and the casing.
In this support construction, the substrate is supported firmly by the quartz sand, and the difference in the thermal expansions of the substrate and the casing is absorbed by the sliding contact between the separation members and the substrate.
However, problems arise when the support construction in the JPP '533 is applied to a substrate formed as a scroll-like cylindrical laminated assembly with a center electrode such as disclosed in the JPP '416.
In the scroll-like cylindrical laminated assembly type substrate, the outer cylindrical surface of the substrate must be connected to the inner surface of the casing so that an electric current can flow through the connection without resistance. However, in the JPP '533, the connection between the substrate and the casing is made only by contact between the substrate and the separating members. Since the electrical resistance of the contact points is high, it is difficult to supply sufficient electricity, for heating the substrate to the activating temperature in a short time, through these contact points.
On the other hand, if the cylindrical outer surface of the substrate is directly fixed to the inner surface of the casing by soldering, for example, to ensure lower electrical resistance in the connection between the substrate and the casing, the movement of the substrate caused by the thermal expansion is hindered. This causes excessive thermal stress in the substrate.
Further, as explained later, in the substrate formed as a scroll-like cylindrical laminated assembly, the substrate, when heated, moves in a radial direction as well as an direction due to thermal expansion of the center electrode.
Therefore, it is difficult to firmly fix the substrate formed as a scroll-like cylindrical laminated assembly to the casing in such manner that a low electrical resistance in the connection between the casing and the substrate is maintained while allowing movement of the substrate relative to the casing.